Recently, there has been growing interest in energy storage technologies in the field of electrochemical devices. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers, and even electric cars, efforts have increasingly been made towards the research and development of batteries. In this aspect, electrochemical devices have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest.
Many companies are currently producing lithium secondary batteries due to their advantages of a higher operating voltage and a higher energy density than a traditional Ni-MH battery using an aqueous electrolyte solution; however, lithium secondary batteries each exhibit different safety characteristics. Particularly, a separator has posed a problem that an internal short circuit is caused by its damage, resulting in an explosion and the like. In a case of a lithium secondary battery using a polyolefin-based material as a separator material, due to the polyolefin-based material having a melting point equal to or less than 200° C., a separator is found to shrink or melt, causing a short circuit between both electrodes when the battery increases in its temperature by an internal and/or external stimuli.
In an attempt to solve the problem in the art, a separator manufactured by coating inorganic particles and a binder onto a porous substrate of polyolefin and the like has been proposed; however, the battery performance degrades due to the binder whereas there is still a demand for improvements in mechanical properties of the separator.
An organic binder polymer compound commonly used as a binder in the art may include, for example, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, and polyimide, singularly or in combination.
Korean Patent Application Laid-open Publication No. 10-2007-0083975A and Korean Patent Application Laid-open Publication No. 10-2007-0019958A proposed a separator in which a porous coating layer formed from a mixture of insulating filler particles and a binder polymer is provided on a porous substrate and a material having a shut-down function is added to the porous coating layer, to enhance the battery safety, and attempts have been made to use engineering plastics having high heat resistance as a separator material, however the prior arts were not satisfactory in terms of mechanical properties, simplification of a manufacturing process, and costs.